Add pid namespace ioctl extension to translate pids between pid names…

…paces

Signed-off-by: Christian Brauner <[email protected]>

README.md

@@ -691,6 +691,13 @@ point that out explicitly and clearly in the associated patches and Cc
    **Use-Case:** Allow for a race free way to go from individual thread to
    thread-group leader pidfd.
 
+* Add a namespace ioctl to translate a pid between pid namespaces.
+
+  **Use-Case:** This makes it possible to e.g., figure out what a given pid in
+  a pid namespace corresponds to in the callers pid namespace. For example, to
+  figure out what the pid of pid 1 inside of the pid namespace is in the
+  ancestors pid namespace.
+
 ## Finished Items
 
 * [x] ability to unmount obstructed mounts. (this means: you have a stack